Doctor blades have been used for years in various different applications. Typically, a doctor blade is used to help separate a material from a piece of equipment. For example, a doctor blade may be used to help remove a web of material from a drum or plate to which the material has been attached. Doctor blades may also be used to clean equipment and/or to impart one or more characteristics into the product being manufactured by the equipment.
In the paper industry, for example, doctor blades are often used to help remove the paper web from a drying drum, such as a Yankee dryer to which the paper web is adhered. In certain papermaking processes, the doctor blade that removes the paper web from the drying drum or any other drum may also be used to crepe the paper to some degree. Such doctor blades are often referred to as “creping blades”. In other papermaking processes, the doctor blade may be used to remove waste material from various pieces of equipment. Such doctor blades are often referred to as “cleaning blades”. The present invention is directed to doctor blades, and more particularly to creping blades and cleaning blades used in papermaking and other web making processes.
The surface profile of bevel surface of the doctor blade, in addition to the geometry of the doctor blade and the particular set-up configuration of the doctor blade with respect to the equipment with which it interacts, can provide for variations in the way the creping blade performs its intended function. For example, it has been discovered that microscopic bumps, machine marks, or surface abnormalities on the creping blade bevel surface can affect the blade's performance and/or the physical characteristics of the material being removed by the blade.
The present prior art methods of improving the performance of doctor blades include changing the geometry of the leading edge of the blade, introducing grooves into the leading edge of the blade, using composite materials, and treating the surface of the blade. Unfortunately, the current methods of improving doctor blades fail to account for the imperfections in the bevel surface of the blade that result from the machine marks left from the processing of the blade itself after conventional finishing.
There are generally two methods to prepare the finished bevel surface of a doctor blade. One is by the conventional use of abrasive media, typically by grinding methods using abrasive stones, wheels, or other abrasive media. Another is to pare material off the surface of the bevel in single or multiple strokes in order to create a working edge or bevel surface. This paring method is known in the art as “skiving.” Bumps and other imperfections are not limited to blades which have been machined in perpendicular to the z-axis of the leading edge of the blade itself; even blades prepared by a skiving process in which the tool marks are parallel with the z-axis of the leading edge of the blade exhibit microscopic bumps along the surface. An emphasis must be placed on machine tool directionality of the effect that it has on the process removing or separating material (for example, a cellulose web) from a rotating drying drum or other equipment. The machine tool mark orientation in papermaking processes are often arranged in the machine direction (MD) (the same direction of web movement) as opposed to the cross direction (CD) (perpendicular to the web movement) because of factors such as lower sheet drag or friction and pitch (residual material from the raw material or processing steps) build up.
Despite the vast amount of information available relating to the manufacture of doctor blades, there is still a need to improve the performance of creping blades and to provide creping blades that can uniquely affect the physical attributes of the materials with which they interact. Due to the way that a creping blade is typically used in the web making process (i.e., the web is removed from a drying roll at high speed by impacting the web against the creping blade), the creping blade can, and often does, cause problems with throughput, tearing of the web, reducing the strength of the web, generating dust, etc.
The present invention provides improved creping blades that address many of the problems presented by currently available creping blades. Specifically, it has been newly discovered that the bevel surface of the blade can be modified to provide unique benefits to the processes and/or materials with which the creping blade interacts. More specifically, it has been found that a step-like polishing process can be used to super-finish the bevel surface of the doctor blade. The super-finished doctor blades exhibit dramatic improvements in performance. Examples of such improvements include, but are not limited to, line speed increases, increased line run times, increased line reliability, improvement in sheet stability, reduction in the amount of dust or other material derived from the web interacting with the blade and/or can provide the product being manufactured with unique physical attributes or improvements of existing desirable attributes not easily attainable by using the doctor blades that are currently commercially available. This includes higher sheet strength or tensile in both the CD and MD direction, and more consistent product attributes especially in the CD direction of the sheet such as caliper and tensiles versus a higher variability with blades that have smoothness imperfections.
Further, the blades of the present invention can provide a less traumatic interaction with the paper web, which can help reduce the amount of material needed to form a particular end product in certain circumstances and/or allow for the use of less expensive materials to produce the desired end product.
The present invention addresses one or more of the disadvantages of currently available creping blades and methods using such creping blades by providing a smoother bevel surface for the creping blade.